Subject: [PATCH v6 20/22] driver core: Allow deferring probes until late init

From: [email-masked] (Rob Herring)
Date: Sat, 26 Sep 2015 13:15:54 -0500

On 09/21/2015 09:03 AM, Tomeu Vizoso wrote:
> Add a field to struct device that instructs the device-driver core to
> defer the probe of this device until the late_initcall level.

This is true until the next patch with moves deferred probe processing
to device_initcall_sync. So disabling this option alone won't totally
revert to current behaviour. I guess patch 21 could be reverted if
necessary.

> 
> By letting all built-in drivers to register before starting to probe, we
> can avoid any deferred probes by probing dependencies on demand.
> 
> Signed-off-by: Tomeu Vizoso <[email-masked]>
> 
> ---
> 
> Changes in v4:
> - Add Kconfig DELAY_DEVICE_PROBES to allow disabling delayed probing in
>   machines with initcalls that depend on devices probing at a given time.
> 
>  drivers/base/Kconfig   | 18 ++++++++++++++++++
>  drivers/base/dd.c      |  7 +++++++
>  include/linux/device.h |  2 ++
>  3 files changed, 27 insertions(+)
> 
> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> index 98504ec99c7d..44b5d33b1f49 100644
> --- a/drivers/base/Kconfig
> +++ b/drivers/base/Kconfig
> @@ -324,4 +324,22 @@ config CMA_ALIGNMENT
>  
>  endif
>  
> +config DELAY_DEVICE_PROBES
> +	bool "Allow delaying the probe of some devices"

I'd like to hide visibility of this behind EXPERT.

> +	default y
> +	help
> +	  Devices can be matched to a driver and probed from the moment they
> +	  are registered, but early during boot their probes are likely to be
> +	  deferred because some dependency isn't available yet because most
> +	  drivers haven't been registered yet.
> +
> +	  Enabling this option allows the device registration code to delay the
> +	  probing of a specific device until device_initcall_sync, when all
> +	  built-in drivers have been registered already.
> +
> +	  In some platforms there may be implicit assumptions about when some
> +	  devices are probed, so enabling this option could cause problems there.
> +
> +	  If unsure, say Y here.
> +
>  endmenu
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 7dc04ee81c8b..269ea76c1a4f 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -417,6 +417,13 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
>  	if (!device_is_registered(dev))
>  		return -ENODEV;
>  
> +#if IS_ENABLED(CONFIG_DELAY_DEVICE_PROBES)

This can be moved to the if.

> +	if (!driver_deferred_probe_enable && dev->probe_late) {
> +		driver_deferred_probe_add(dev);
> +		return 0;
> +	}
> +#endif
> +
>  	pr_debug("bus: '%s': %s: matched device %s with driver %s\n",
>  		 drv->bus->name, __func__, dev_name(dev), drv->name);
>  
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 8e7b806f0744..e64f4c7e243d 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -744,6 +744,7 @@ struct device_dma_parameters {
>   *
>   * @offline_disabled: If set, the device is permanently online.
>   * @offline:	Set after successful invocation of bus type's .offline().
> + * @probe_late:	If set, device will be probed in the late initcall level.
>   *
>   * At the lowest level, every device in a Linux system is represented by an
>   * instance of struct device. The device structure contains the information
> @@ -828,6 +829,7 @@ struct device {
>  
>  	bool			offline_disabled:1;
>  	bool			offline:1;
> +	bool			probe_late:1;
>  };
>  
>  static inline struct device *kobj_to_dev(struct kobject *kobj)
>